A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
An assembly comprising a conditioning system and at least one object is known from the art. For example, European patent EP 0 498 499 illustrates, in FIG. 18 thereof, a part of a lithographic apparatus. The apparatus includes an interferometer system and a space in which respective interferometer beams propagate, near a movable substrate table. A constant, desirably laminar stream of gas (e.g., air) is passed through this space, to obtain a greater accuracy of the interferometer system. It is well known that, for interferometry beams propagating through a gas volume (such as air), the gas in the volume should be of extreme purity and have a uniform temperature to provide a substantially constant gas index of refraction for the propagating beam. Small changes in the gas refractive index may produce an unacceptable interferometry error. Both the purity and the temperature of the supplied gas can be controlled. In the case of air, the air is, for example, of purity class 1 and its temperature is, for example, stable to within 0.10 degrees C. or better.